CSIs are not detectives as this is exceedingly rare in real life, as they are actually classed as civilian employees. It is considered an inappropriate and improbable practice to allow CSI personnel to be involved in detective work as it would compromise the impartiality of scientific evidence and would be impracticably time-consuming. See more
[looking at evidence from a murder case
I can't wrap my head around it, Mac. You get up, you go to work, see the people that you know, you talk, you laugh. You're living your life, then suddenly, boom. It's just over. Just like that, and you never even saw it coming.